Variable optical attenuator

ABSTRACT

A method of controllably attenuating a beam of light coupled into a port includes directing the beam of light against a mirror, and controlling an orientation of the mirror such that a predetermined fraction of the beam of light is coupled into the port. The predetermined fraction is less than a maximum fraction corresponding to optimal coupling of the beam of light into the port. The method may be implemented with a variable optical attenuator including a first port, a second port, a mirror located to direct light output by the first port to the second port, and a controller coupled to the mirror to align it such that the predetermined fraction of light is coupled into the second port. The method may also be implemented with an optical switch.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to the following U.S. PatentApplications: Attorney Docket No. M-10967 US, Attorney Docket No.M-11418 US, Attorney Docket No. M-11419 US, Attorney Docket No. M-11502US, Attorney Docket No. M-11745 US, and U.S. patent application Ser. No.09/779,189 entitled “A Microelectromechanical Mirror,” filed Feb. 7,2001, all of which are assigned to the assignee of the present inventionand incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to optical fiber cross-connectswitching. More particularly, it relates lo load balancing in DenseWavelength Division Multiplexing optical cross-connect systems.

[0004] 2. Description of the Related Art

[0005] Associated with the information revolution is a need to increaseby many orders of magnitude the rate of information transfer. This canbe accomplished with optical fibers and the method of Dense WavelengthDivision Multiplexing (DWDM), in which many wavelength channels, eachincluding a different narrow band of wavelengths of light and eachcarrying different information, are multiplexed onto a single opticalfiber using an optical multiplexer. Optical signals carried on thevarious wavelength channels may be separated at the output of theoptical fiber with an optical demultiplexer.

[0006] Optical fiber cross-connect switches may be used to direct theoptical signals on some or all of the wavelength channels on aparticular optical fiber to other optical fibers. Such optical fibercross-connect switches include those described in U.S. PatentApplication Attorney Docket No. M-10967 US, U.S. Patent ApplicationAttorney Docket No. M-11418 US, and U.S. Patent Application AttorneyDocket No. M-11745 US, all of which are incorporated herein by referencein their entirety. Hence, optical signals on the various wavelengthchannels on an optical fiber may have originated at separate locationsand traveled different distances in optical fiber. Since light isattenuated during transmission through optical fiber by an amounttypically proportional to the distance traveled in optical fiber, thevarious wavelength channels on an optical fiber may carry differentpower levels.

[0007] Optical amplifiers such as Erbium Doped Fiber Amplifiers (EDFA)can amplify a wide wavelength band (spanning many wavelength channels),and thus compensate for transmission losses in optical fibers. If thepower levels on the various wavelength channels carried by the opticalfiber are not nearly equal at the input to the optical amplifier,however, the wavelength channel or channels of highest power maysaturate the gain. Under such circumstances, the lower power wavelengthchannels might not be sufficiently amplified.

[0008] A variable optical attenuator is an optical device with which theamplitude or power level of an input optical signal may be attenuated bya variable amount to provide an output optical signal of a desiredamplitude or power level. The power levels of the various wavelengthchannels on an optical fiber may be substantially equalized in a “loadbalancing” or “load equalization” process in which each wavelengthchannel is routed through a separate variable optical attenuator.Variable optical attenuators are described, for example, in U.S. Pat.Nos. 5,864,643 and 6,130,984. These devices require the insertion ofadditional hardware into an optical network. The additional hardware maybe expensive, requires additional physical space, and may introduceunwanted attenuation of the optical signals.

[0009] It would be desirable to incorporate the function of a variableoptical attenuator into an optical network without the insertion ofadditional optical elements.

SUMMARY

[0010] A method of controllably attenuating a beam of light coupled intoa port in accordance with an embodiment of the present inventionincludes directing the beam of light against a mirror, and controllingan orientation of the mirror such that a predetermined fraction of thebeam of light is coupled into the port. The predetermined fraction isless than a maximum fraction corresponding to optimal coupling of thebeam of light into the port. In one embodiment, this method isimplemented with a variable optical attenuator including a first port, asecond port, a mirror located to direct light output by the first portto the second port, and a controller coupled to the mirror to align itsuch that the predetermined fraction of light is coupled into the secondport. The ports may be or include optical fibers.

[0011] In one implementation, the variable optical attenuator includes asecond mirror located to direct to the second port light output by thefirst port and reflected by the first mirror. The controller is alsocoupled to the second mirror to align it such that the predeterminedfraction of light is coupled into the second port. Use of twocontrollable mirrors in the optical path of the light beam allowsindependent control of the position and angle of incidence of the lightbeam on the second port.

[0012] Control of the mirror or mirrors in the variable opticalattenuator may be accomplished by numerous methods. In oneimplementation, the power of light coupled into the second port ismeasured, and an orientation of a mirror is controlled to maintain thepower at a predetermined level. In another implementation, anorientation of a mirror corresponding to the predetermined fractiondescribed above is selected from a look-up table. In anotherimplementation, an alignment beam of light is directed against a mirror,and the orientation of the mirror is controlled to direct the alignmentbeam to a predetermined position on a position sensing detector. Thepredetermined position corresponds to the predetermined fractiondescribed above.

[0013] In another embodiment, a variable optical attenuator includes afirst plurality of ports, a second plurality of ports, a first pluralityof mirrors disposed on a first surface, a second plurality of mirrorsdisposed on a second surface, and a controller coupled to align each ofthe first plurality of mirrors and each of the second plurality ofmirrors such that predetermined fractions of light output by the firstplurality of ports are coupled into separate ones of the secondplurality of ports. The predetermined fractions are less than maximumfractions corresponding to optimal coupling of light output by the firstplurality of ports into the second plurality of ports. This embodimentmay be employed, for example, to load balance DWDM wavelength channels.

[0014] A method of equalizing the power levels of (load balancing) aplurality of channels multiplexed on an optical fiber in accordance withan embodiment of the present invention includes demultiplexing thechannels from the optical fiber to form a plurality of beams of light,with each beam of light formed from a separate channel, measuring thepower level of each channel, directing each of the beams of lightagainst a separate one of a plurality of mirrors, and controlling anorientation of one of the mirrors such that a predetermined fraction ofthe beam of light directed against that mirror is coupled into a port.The predetermined fraction is less than a maximum fraction correspondingto optimal coupling of the beam of light into the port.

[0015] Variable optical attenuators in accordance with embodiments ofthe present invention may be implemented in optical cross-connectswitches. In such embodiments, the ports and mirrors of the variableoptical attenuator may also support switching functions in the opticalcross-connect switch. Optical cross-connect switches are typicallydesigned and operated to achieve minimum insertion loss for all opticalsignals coupled into the switch. The inventors have recognized, however,that variable attenuation can be accomplished by separately controllingthe insertion loss for the various optical signals by controllablymisaligning mirrors used to switch the optical signals. Hence, thefunction of one or more variable optical attenuators may beadvantageously integrated into an optical network without the insertionof additional optical elements.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 illustrates schematically a variable optical attenuator inaccordance with an embodiment of the present invention.

[0017]FIG. 2 illustrates schematically a portion of a variable opticalattenuator in accordance with the embodiment of FIG. 1.

[0018]FIG. 3 illustrates schematically a variable optical attenuator inaccordance with another embodiment of the present invention.

[0019]FIG. 4 illustrates schematically an optical fiber cross-connectswitch in which is implemented a variable optical attenuator inaccordance with an embodiment of the present invention.

[0020]FIG. 5 is a plot showing attenuation of the optical power coupledinto an optical fiber versus misalignment of a light beam with respectto the optical fiber in accordance with an embodiment of the presentinvention.

[0021] Like reference numbers in the various figures denote same partsin the various embodiments. Dimensions in the figures are notnecessarily to scale.

DETAILED DESCRIPTION

[0022] A variable optical attenuator in accordance with embodiments ofthe present invention variably attenuates light coupled into an opticalfiber by controlled misalignment of one or more mirrors directing thelight to the fiber. A number of embodiments will be described in whichone or more optical signals are variably attenuated, and in whichcontrolled misalignment of one or more mirrors is accomplished using,for example, measurements of the power of the attenuated optical signalsor measurements of the position of control light beams separate from theoptical signals to be attenuated.

[0023] Referring to FIG. 1, optical fiber 2 carries light to beattenuated by a controlled amount in a variable optical attenuator 1 inaccordance with an embodiment of the present invention. As isconventional in DWDM, optical fiber 2 may carry light having a pluralityof wavelengths. In one implementation, the light carried by opticalfiber 2 has wavelengths near about 1310 nanometers (nm) or about 550 nm.Optical fiber 2 is, for example, a conventional Corning, IncorporatedSMF-28 single mode optical fiber having a core diameter of about 8microns (μm) and a cladding diameter of about 125 μm. Other opticalfibers suitable for optical communications applications may also beused.

[0024] Optical fiber 2 outputs a diverging cone of light which is, forexample, collimated or weakly focused by lens 6 to form light beam 8.Lens 6 is, for example, a conventional plano-convex lens formed from BK7 optical glass and having a focal length of about 4 millimeters (mm).Light beam 8 is incident on beam splitter 10, which divides light beam 8into light beam 8 a incident on mirror 12 and light beam 8 b incident onphotodetector 14. Photodetector 14 is, for example, a conventionalInGaAs photodiode. Suitable InGaAs photodiodes are available from, forexample, Hamamatsu Corporation of Bridgewater, N.J. and Telcom DevicesCorporation of Camarillo, Calif.

[0025] In one implementation, beam splitter 10 is a cube beam splitterformed from BK 7 optical glass and having a dielectric coating with areflectivity of about 2% at infrared wavelengths of about 1200 nm toabout 1700 nm. In another implementation, beam splitter 10 is a dichroiccube beam splitter formed from BK 7 optical glass and having adielectric coating with a reflectivity of about 2% at infraredwavelengths of about 1200 nm to about 1700 nm and a reflectivity ofabout 40% to about 60%, preferably about 50%, at wavelengths of about600 nm to about 850 nm. Such beam splitters are available, for example,from Harold Johnson Optical Laboratories, Inc. of Gardena, Calif.Suitable coatings for the beam splitter may be obtained, for example,from ZC&R Coatings For Optics, Inc. of Torrance, Calif.

[0026] The reflectivity of such a dichroic beam splitter may beselected, for example, to allow at least partial separation ofwavelengths of light used in telecommunications (e.g., 1200 nm-1700 nm)from another range (e.g., 600 nm-850 nm) of non-telecommunicationwavelengths used for control light beams used in some embodiments asdescribed below.

[0027] Referring again to FIG. 1, mirror 12 directs light beam 8 a,incident from beam splitter 10, through (optional) beam splitter 22, ifit is present, to lens 24. In some implementations, lens 6 focuses lightbeam 8 a to a waist at a location along light beam 8 a between lens 6and lens 24. Such focusing can maintain a relatively small diameter oflight beam 8 a throughout variable optical attenuator and thus reduceuncontrolled optical loss from, e.g., diffraction.

[0028] Mirror 12 is coupled to actuator 16, which is controlled bycontrol system 18 with electrical signals provided via electrical line20 to orient mirror 12 in a range of arbitrary directions (dθ,dφ). Thisrange of orientations allows mirror 12 to direct light beam 8 a ontolens 24 at a range of controlled angles with respect to optical axis 28(FIGS. 2A-2C) of lens 24 and to a range of controlled positions onsurface 25 of lens 24.

[0029] In one implementation, mirror 12 and actuator 16 are,respectively, a micro-electro-mechanical systems (MEMS) micro mirror anda MEMS actuator as described, for example, in U.S. patent applicationSer. No. 09/779,189 incorporated herein by reference in its entirety.Other micro mirrors may also be used. In such an implementation, mirror12 may be a freely rotatable MEMS micro mirror actuated by, for example,electrostatic, electromagnetic, piezoelectric, or thermal actuationmeans. Mirror 12 may be, for example, approximately elliptical withmajor and minor diameters of about 1.0 mm and about 0.9 mm,respectively. Control system 18 may be, for example, a control systemfor a MEMS based optical switch such as, for example, control systemsdisclosed in U.S. Patent Application Attorney Docket No. M-11419 US andU.S. Patent Application Attorney Docket No. M-11502 US, both of whichare incorporated herein by reference in their entirety.

[0030] In other implementations, mirror 12 may be a conventional mirrorhaving a metal or dielectric coating highly reflective at wavelengths ofabout 1200 nm to about 1700 nm. Actuator 16 may be a conventional mirrormount actuated by, for example, conventional stepper motors orconventional piezoelectric actuators. Control system 18 may include, forexample, a microprocessor and a conventional stepper motor driver or aconventional piezoelectric driver.

[0031] Lens 24 focuses light beam 8 a, incident from mirror 12, ontooptical fiber 26. In one implementation, for example, surface 25 ofoptical fiber 26 is approximately at the focal plane of lens 24.Referring to FIGS. 2A-2C, lens 24 is positioned with its optical axis 28approximately centered on the core 26 a of optical fiber 26. Lens 24 maybe, for example, a conventional plano-convex lens formed from BK 7optical glass and having a focal length of about 4 mm. Optical fiber 26includes cladding 26 b surrounding core 26 a.

[0032] One of ordinary skill in the art will recognize that lens 24 maycouple light beam 8 a into a core (e.g., fundamental) optical mode ofoptical fiber 26 and/or into a cladding mode of optical fiber 26. Thepower distribution of light in a core mode of optical fiber 26 isconcentrated in core 26 a, although a portion of the power 10distribution of such a core mode propagates in cladding 26 b. Lightcoupled into a core mode can propagate long distances with littleattenuation. In contrast, the power distribution of a cladding mode ofoptical fiber 26 is concentrated in cladding 26 b.

[0033] Only that portion of light beam 8 a incident on core 26 a ofoptical fiber 26 at angles with respect to optical axis 28 less than theacceptance angle (determined by the refractive indices of core 26 a andcladding 26 b) of optical fiber 26 will be efficiently coupled into acore mode of optical fiber 26. Hence, the fraction of light beam 8 acoupled into a core mode of optical fiber 26 depends on the location atwhich light beam 8 a is incident on lens 24 and the angle that lightbeam 8 a makes with respect to optical axis 28. In FIG. 2A, for example,light beam 8 a is incident on the approximate center of lens 24approximately parallel to optical axis 28 and focused entirely onto core26 a within the acceptance angle θ_(A) indicated by dashed lines 27.Thus, light beam 8 a is approximately aligned for maximum coupling intoa core mode of optical fiber 26. The acceptance angle of optical fiber26 in air may be, for example, about 7.5° (numerical aperture of about0.13). One of ordinary skill in the art will recognize that the maximumoptical power coupled into a core mode of optical fiber 26 is typicallyless than the total optical power of light beam 8 a as a result of, forexample, Fresnel reflection losses at surface 25.

[0034] In contrast, in FIG. 2B, light beam 8 a is incident on lens 24 atan angle θ with respect to optical axis 28 sufficiently large that lightbeam 8 a misses core 26 a and is focused entirely onto cladding 26 b.Hence, little or none of light beam 8 a is coupled into a core mode ofoptical fiber 26. Light coupled into a cladding mode of optical fiber 26is subsequently removed, for example, by a conventional cladding modestripper 30 (FIG. 1). One of ordinary skill in the art will recognizethat light coupled into cladding modes of an optical fiber is typicallyrapidly attenuated during transmission, particularly if the opticalfiber is coiled or otherwise bent. Thus, in other implementationscladding mode stripper 30 is not used. Since light coupled into acladding mode of optical fiber 26 is subsequently removed or otherwiseattenuated, light described herein as being coupled into optical fiber26 refers to light coupled into a core mode of optical fiber 26 ratherthan into, for example, a cladding mode of optical fiber 26.

[0035]FIGS. 2A and 2B show alignments of light beam 8 a resulting in,respectively, approximately minimum attenuation and approximatelymaximum attenuation of the light coupled into optical fiber 26. Controlsystem 18 may control the orientation of mirror 12 to achieve alignmentsof light beam 8 a interrnediate between those of FIGS. 2A and 2B, andthus vary the attenuation of the light coupled into optical fiber 26between the approximate minimum and approximate maximum levels ofattenuation. In FIG. 2C, for example, light beam 8 a is incident on lens24 at an angle θ with respect to optical axis 28 smaller than that ofFIG. 2B and focused to overlap both core 26 a and cladding 26 b ofoptical fiber 26. A fraction of light beam 8 a focused onto surface 25at angles less than the acceptance angle of optical fiber 26 will becoupled into a core mode of optical fiber 26. Another fraction of lightbeam 8 a may be coupled into a cladding mode of optical fiber 26 andsubsequently removed as described above.

[0036] Referring to FIG. 5, curve 29 is a plot, for one implementation,of the attenuation of the optical power coupled into optical fiber 26 asa function of the offset of the center of light beam 8 a at surface 25from the center of core 26 a of optical fiber 26. In thisimplementation, lens 24 has a focal length of about 4 mm, light beam 8 ahas a diameter of about 0.8 mm at lens 24, core 26 a of optical fiber 26has a diameter of about 8 μm, and surface 25 is approximately at thefocal plane of lens 24. As curve 29 indicates, an offset of about 18 μmbetween the center of the focused beam and the center of optical fiber26 in this implementation results in an attenuation of about 60 decibels(dB). This offset corresponds to a misalignment of light beam 8 a withrespect to optical axis 28 (FIGS. 2A-2C) of about 0.25°. Such amisalignment of light beam 8 a can be achieved with a misalignment ofmirror 12 of about 0.125° since the angular displacement of light beam 8a is twice that of mirror 12.

[0037] In many optical communication applications the maximum opticalattenuation required is about 30 dB. The slope of curve 29 at about 30dB attenuation, represented by line 31, is about 1 dB of attenuation per0.22 μm of offset. This corresponds to about 3 dB per 0.01° misalignmentof light beam 8 a with respect to optical axis 28. Hence, control of theorientation of mirror 12 with a resolution of better than about 0.005°allows control of the attenuation of the power of light coupled intooptical fiber 26 with a resolution better than about 3 dB. Such angularresolution may be achieved, for example, in optical fiber cross-connectswitches described in U.S. Patent Application Attorney Docket No.M-10967 US, U.S. Patent Application Attorney Docket No. M-11418 US, andU.S. Patent Application Attorney Docket No. M-11745 US. Of course, theresolution with which the attenuation is controlled improves as theangular resolution with which the mirror is controlled is improved.Attenuation curves for other implementations are similar to curve 29.

[0038] Controlled misalignment of mirror 12 to attenuate light coupledinto optical fiber 26 may be accomplished by numerous methods. Referringagain to FIG. 1, control system 18 receives electrical signalscorresponding to the optical power in light beam 8 b (and thus in lightbeam 8 a) from photodetector 14 via electrical line 32. Control system18 determines from these electrical signals the amount by which lightbeam 8 a is to be attenuated. In one embodiment, (optional) conventionalfiber coupler 34 directs a portion of the light coupled into opticalfiber 26 to (optional) photodetector 36. Photodetector 36, which may bea conventional InGaAs photodiode, provides a signal corresponding to theoptical power coupled into optical fiber 26 to control system 18 viaelectrical line 38. Control system 18 controls the orientation of mirror12 such that the electrical signals provided by photodetector 36indicate that the light coupled into optical fiber 26 is attenuated tothe desired power level. Hence, in this embodiment control system 18,actuator 16, mirror 12, and photodetector 36 form a feedback loop bywhich attenuation of the light coupled into optical fiber 26 iscontrolled.

[0039] In another embodiment, a look-up table stored in a computerreadable medium (memory 18 a) in control system 18 relates theorientation of mirror 12 to the attenuation of the light coupled intooptical fiber 26. In this embodiment, control system 18 determines fromthe electrical signals received from photodetector 14 the amount bywhich light beam 8 a is to be attenuated, reads the required orientationof mirror 12 from the look-up table (which contains informationcorrelating the amount of attenuation to the mirror's orientation), andcontrols actuator 16 to orient mirror 12 accordingly. The look-up tablemay be generated by measuring, with photodetectors 14 and 36, forexample, the attenuation of light coupled into optical fiber 26 for eachof a series of different orientations of mirror 12.

[0040] In other embodiments, controlled misalignment of mirror 2 isaccomplished using measurements of the position of one or more controllight beams separate from the optical signals to be attenuated. In theseembodiments, mirror 12 may be controlled, for example, using methodssimilar or identical to methods for controlling the orientations ofmirrors in an optical fiber cross-connect switch disclosed in thefollowing U.S. Patent Applications: Attorney Docket No. M-10967 US,Attorney Docket No. M-11418 US, Attorney Docket No. M-11419 US, AttorneyDocket No. M-11502 US, and Attorney Docket No. M-11745 US.

[0041] In one implementation, for example, laser 40 (FIG. 1) outputscontrol light beam 42 incident on dichroic beam splitter 10. In someimplementations, the wavelength of control light beam 42 is a wavelengthnot typically used in telecommunications. In one implementation, forexample, laser 40 is a conventional laser diode emitting light having awavelength of about 660 nm. Dichroic beam splitter 10 reflects lightbeam 42 lo mirror 12, which directs control light beam 42 to dichroicbeam splitter 22. Dichroic beam splitter 22 reflects control light beam42 to position sensing detector 44. Thus, the position at which controllight beam 42 is incident on position sensing detector 44 is determinedby the orientation of mirror 12. Dichroic beam splitter 22 is, forexample, substantially identical to dichroic beam spliner 10. Suitableposition sensing detectors are available, for example, from UDT Sensors,Inc. of Hawthorne, Calif., and from Pacific Silicon Sensor, Inc. ofWestlake Village, Calif.

[0042] Position sensing detector 44 provides a signal indicating theposition at which control light beam 42 is incident on it lo controlsystem 18 via electrical line 46. A look-up table stored in memory 8 ain control system 18 relates the attenuation of the light coupled intooptical fiber 26 lo the position at which control light beam 42 isincident on position sensing detector 44. In this implementation,control system 18 determines from the electrical signals received fromphotodetector 14 the amount by which light beam 8 a is to be attenuated,determines from the look-up table the corresponding position on positionsensing detector 44 to which control light beam 42 is to be directed,and controls the orientation of mirror 12 to direct control light beam42 to that position. The look-up table used in this implementation maybe generated by measuring the attenuation of light coupled into opticalfiber 26 and the position at which control light beam 42 is incident onposition sensing detector 44 for each of a series of orientations ofmirror 12.

[0043] Variable optical attenuator 1 of FIG. 1 includes only one mirror(mirror 12) having an orientation controlled by control system 18 in arange of directions (dθ,dφ). The angle of incidence of light beam 8 a onsurface 25 and the location on surface 25 at which light beam 8 a isincident (FIGS. 2A-2C) cannot be independently controlled with thissingle controlled mirror.

[0044] In other embodiments, light beam 8 a is directed to optical fiber26 by two or more mirrors having orientations controlled by controller18. For example, variable optical attenuator 47 shown in FIG. 3includes, in addition to the elements shown in FIG. 1, mirror 48 coupledto actuator 50. Actuator 50 is controlled by control system 18 withelectrical signals provided via electrical line 52 to orient mirror 48in a range of arbitrary directions (dθ,dφ). Either or both of mirrors 12and 48 can be controllably misaligned, by the methods described above,to variably attenuate light coupled into optical fiber 26. The use oftwo controllable mirrors in the optical path of light beam 8 a allowsindependent control of the angle of incidence of light beam 8 a onsurface 25 and the location on surface 25 at which light beam 8 a isincident. This may result in better control of light coupled intooptical fiber 26. In some embodiments, lens 6 focuses light beam 8 a toa waist at a location along light beam 8 a between mirror 12 and mirror48. Such focusing can maintain a relatively small diameter of light beam8 a throughout variable optical attenuator 47 and thus reduceuncontrolled optical loss.

[0045] Variable optical attenuators in accordance with embodiments ofthe present invention may be implemented within optical fibercross-connect switches such as those described in U.S. PatentApplication Attorney Docket No. M-10967 US, U.S. Patent ApplicationAttorney Docket No. M-1418 US, and U.S. Patent Application AttorneyDocket No. M-11745 US. In particular, mirrors 12 and 48 (FIG. 3) may bemirrors in an optical fiber cross-connect switch oriented to couplelight from an input port (optical fiber 2) to an output port (opticalfiber 26). Although FIGS. 1 and 3 show only a single input optical fiber2 and a single output optical fiber 26, an optical fiber cross-connectswitch within which a variable optical attenuator is implemented inaccordance with an embodiment of the present invention typically has aplurality of inputs and a plurality of outputs. In a typical opticalpath through such a switch, light entering the switch through an inputport is incident on a corresponding first micro-mechanical mirror in afirst two dimensional array of micro-mechanical mirrors. The firstmicro-mechanical mirror, which can be oriented in a range of arbitrarydirections (dθ,dφ), is tilted to direct the light to a secondmicro-mechanical mirror in a second two dimensional array ofmicro-mechanical mirrors. The second micro-mechanical mirror, which canalso be oriented in a range of arbitrary directions (dθ,dφ), is tiltedto direct the light to a corresponding output port and hence out of theswitch.

[0046] The light may be switched from the output port to which it isinitially directed to another output port by reorienting the firstmicro-mechanical mirror to direct the light to another (i.e., a third)micro-mechanical mirror in the second array of micro-mirrors, andorienting the third micro-mechanical mirror to direct the light to itscorresponding output port. The micro-mechanical mirrors may be optimallyaligned to couple a maximum amount of light into an output port, orcontrollably misaligned to variably attenuate the light coupled into anoutput port. Advantageously, a variable optical attenuator function canbe thereby added to an optical network without the insertion ofadditional optical elements, as power sensing functions such as thoseprovided by beam spliner 10 and photodetector 14 (FIG. 1) are typicallypresent in such switches for control and monitoring purposes. In thisway, light entering the switch at any input port can be directed to anyoutput port with the proper attenuation.

[0047] Referring to FIG. 4, for example, optical fiber cross-connectswitch 53 (substantially similar to those described in U.S. PatentApplication Attorney Docket No. M-10967 US, U.S. Patent ApplicationAttorney Docket No. M-11418 US, and U.S. Patent Application AttorneyDocket No. M-11745 US) routes light carried by any one of N inputoptical fibers 54-1-54-N to any one of N output optical fibers 56-1-56-Nand also performs variable optical attenuation functions in accordancewith an embodiment of the present invention. In the implementation shownin FIG. 4, the number of input optical fibers equals the number ofoutput optical fibers. Other implementations include N input opticalfibers and P output optical fibers, with either N<P or N>P. Typically,both N and P are greater than about 1000. In one implementation, forexample, N is about 1200 and P=N.

[0048] Optical fibers 54-1-54-N output diverging cones of light whichare collimated or weakly focused by, respectively, lenses 60-1-60-N toform, respectively, light beams 62-1-62-N incident on beam splitter 10.Although for convenience of illustration optical fibers 54-1-54-N areshown in FIG. 4 arranged in a single row, typically the ends of opticalfibers 54-1-54-N are arranged in a two dimensional array. Lenses60-1-60-N may be identical to lens 6 of FIG. 1. Alternatively, lenses60-1-60-N may be lenslets (small lenses) arranged in a two dimensionallenslet array sometimes called a microlens array.

[0049] Beam splitter 10 divides light beams 62-1-62-N into light beams66-1-66-N and light beams 68-1-68-N. Light beams 66-1-66-N are incidenton, respectively, lenses 70-1-70-N which focus them onto separate spotson input sensor 72. Input sensor 72 detects the intensity of each oflight beams 66-1-66-N and provides corresponding electrical signals tocontrol system 18 via bus 74. Input sensor 72 is, for example, aSU128-1.7RT infrared camera having a 128×128 pixel array available fromSensors Unlimited, Inc. of Princeton, N.J.

[0050] Light beams 68-1-68-N are incident on, respectively, micromirrors 76-1-76-N of micro mirror array 76. Typically, micro mirrors76-1-76-N are arranged in a two dimensional array corresponding to thatof lenses 60-1-60-N and optical fibers 54-1-54-N. In someimplementations the pitch of micro mirrors 76-1-76-N in a directionalong mirror array 76 parallel to the planes of incidence of light beams68-1-68-N (defined by light beams 68-1-68-N and axes normal to mirrorarray 76 at the points at which the light beams intersect mirror array76) is elongated compared to the corresponding pitch of lenses 60-1-60-Nsuch that light beams 68-1-68-N are incident approximately centered onmicro mirrors 76-1-76-N. The orientations of micro mirrors 76-1-76-N areindividually controllable over a range of arbitrary angles (dθ,dφ) bycontrol system 18 with electrical signals transmitted via bus 78. Micromirror array 76 is, for example, a MEMS micro mirror array described inU.S. patent application Ser. No. 09/779,189.

[0051] In the illustrated embodiment, micro mirrors 76-1-76-N reflectlight beams 68-1-68-N, respectively, onto fold mirror 80. Fold mirror 80reflects light beams 68-1-68-N onto micro mirror array 82. Micro mirrorarray 82 includes N micro mirrors 82-1-82-N. The orientations of micromirrors 82-1-82-N are individually controllable over a range ofarbitrary angles (dθ,dφ) by control system 18 with electrical signalstransmitted via bus 83. In one implementation, micro mirror arrays 76and 82 are substantially identical.

[0052] In the illustrated embodiment each of micro mirrors 76-1-76-N iscontrollable to reflect a light beam incident on it from thecorresponding one of optical fibers 54-1-54-N to any one of micromirrors 82-1-82-N via fold mirror 80. Hence, control system 18 cancontrol the orientations of micro mirrors 76-1-76-N to reflect, via foldmirror 80, any one of light beams 68-1-68-N onto the approximate centerof any one of micro mirrors 82-1-82-N. For example, FIG. 4 shows lightbeams 68-1, 68-2, and 68-N reflected to, respectively, micro mirrors82-K. 82-J, and 82-I. Micro mirrors 82-I. 82-J, and 82-K, which need notbe adjacent to one another, may be any of micro mirrors 82-1-82-N. Inother embodiments micro mirrors 76-1-76-N are controllable to reflectlight beams 68-1-68-N to any one of micro mirrors 82-1-82-N without theuse of a fold mirror such as fold mirror 80. In some such embodiments,for example, micro mirrors 76-1-76-N may reflect light beams 68-1-68-Ndirectly to any one of micro mirrors 82-1-82-N.

[0053] Control system 18 controls the orientations of micro mirrors82-1-82-N to reflect the light beams incident on them from micro mirrorarray 76 to, respectively, lenses 84-1-84-N. FIG. 4 shows micro mirrors82-I, 82-J, and 82-K reflecting, respectively, light beams 68-N, 68-2,and 68-1 to, respectively, lenses 84-I, 84-J, and 84-K. It should beunderstood, however, that each particular one of micro mirrors 82-1-82-Nis controlled to reflect whichever one of light beams 68-1-68-N isincident on it to the lens 84-1-84-N corresponding lo that particularmicro mirror. For example, micro mirror 82-1 is controlled to reflectwhichever one of light beams 68-1-68-N is incident on it to lens 84-1.

[0054] Lenses 84-1-84-N focus light beams reflected by, respectively,micro mirrors 82-1-82-N onto, respectively, optical fibers 56-1-56-N.Lenses 84-1-84-N may be, for example, substantially identical to lenses60-1-60-N.

[0055] Control system 18 determines from the electrical signals providedby input sensor 72 the amount by which light beams 60-1-60-N must beattenuated, and controls the orientation of micro mirrors 76-1-76-N and82-1-82-N by, for example, the methods disclosed above (e.g., usingcontrol light beams and position sensing detectors) to variablyattenuate and/or switch the light beams between output optical fibers56-1-56-N.

[0056] In one embodiment, variable attenuation functions of opticalcross-connect switch 53 are used to substantially equalize (loadbalance) the power levels of M Dense Wavelength Division Multiplexingwavelength channels on a single optical fiber, where M≦N (N the numberof input ports). The wavelength channels are demultiplexed from theoptical fiber with a conventional optical demultiplexer, and eachcoupled onto a separate one of M of the input optical fibers 54-1-54-N.Control system 18 determines the power levels of the M wavelengthchannels from the electrical signals it receives from input sensor 72,and controls mirror arrays 76 and 82 to route each of the M light beamscorresponding to the various wavelength channels to a separate one of Mof the output optical fibers 56-1-56-N. The lowest power wavelengthchannel is routed to its corresponding output optical fiber with, forexample, approximately minimal attenuation. The power levels of theother wavelength channels are attenuated, for example, to approximatelythat of the lowest power wavelength channel by controllably misaligningthe micro mirrors of mirror arrays 76 and 82 as described above. Aconventional optical multiplexer coupled to the M output optical fibersthen multiplexes the wavelength channels onto a single optical fiber.

[0057] While the present invention is illustrated with particularembodiments, the invention is intended to include all variations andmodifications falling within the scope of the appended claims.

1-32. (canceled)
 33. A method, comprising: directing the beam of lightagainst a mirror; and controlling an orientation of the mirror such thata predetermined fraction of the beam of light is coupled into the port;wherein the predetermined fraction is less than a maximum fractioncorresponding to optimal coupling of the beam of light into the port.34. The method of claim 3, wherein the port includes an optical fiber.35. The method of claim 3, wherein the port is included in an opticalcross-connect switch.
 36. The method of claim 3, wherein the mirror isincluded in an optical cross-connect switch.
 37. The method of claim 33,further comprising measuring a power of the beam of light, anddetermining from the power an amount by which to attenuate the beam oflight.
 38. The method of claim 33, further comprising measuring a powerof light coupled into the port, and controlling the mirror to maintainthe power at a predetermined level.
 39. The method of claim 33, furthercomprising selecting from a look-up table and orientation of the mirrorcorresponding to the predetermined fraction.
 40. The method of claim 33,wherein the beam of light is a first beam of light, further comprisingdirecting another beam of light against the mirror, and controlling theorientation of the mirror to reflect the other beam of light to apredetermined position on a position sensing detector, the predeterminedposition corresponding to the predetermined fraction of the first beamof light.
 41. The method of claim 40, further comprising selecting thepredetermined position from a look-up table.
 42. The method of claim 33,wherein the mirror is a first mirror, further comprising reflecting thebeam of light to a second mirror and controlling an orientation of thesecond mirror such that the predetermined fraction of the beam of lightis coupled into the port.
 43. A variable optical attenuator comprising:a first plurality of ports; a second plurality of ports; a secondplurality of mirrors disposed on a first surface; a second plurality ofmirrors disposed on a second surface; and a controller coupled to aligneach of the first plurality of mirrors and each of the second pluralityof mirrors such that predetermined fractions of light output by thefirst plurality of ports are coupled into separate ones of the secondplurality of ports, wherein at least a subset of the predeterminedfractions are less than maximum fractions corresponding to optimalcoupling of light output by the first plurality of ports into the secondplurality of ports.
 44. The optical switch of claim 43, wherein thefirst plurality of ports and the second plurality of ports each includesgreater than about 1000 ports.
 45. The optical switch of claim 43,wherein the first plurality of mirrors and the second plurality ofmirrors each includes greater than about 1000 mirrors.
 46. The opticalswitch of claim 43, wherein the controller controls an orientation ofeach of the first plurality of mirrors and each of the second pluralityof mirrors with an angular resolution better than about 0.005°.
 47. Amethod of equalizing the power levels of a plurality of channelsmultiplexed on an optical fiber, the method comprising: demultiplexingthe channels from the optical fiber to form a plurality of beams oflight, each beam of light formed from a separate channel; measuring thepower level of each channel; directing each of the beams of lightagainst a separate one of a plurality of a mirrors; and controlling anorientation of one of the mirrors such that a predetermined fraction ofthe beam of light directed against the one of the mirrors is coupledinto a port, the predetermined fraction less than a maximum fractioncorresponding to optimal coupling into the port.
 48. The method of claim47, wherein each of the channels includes a separate range ofwavelengths of light.
 49. The method of claim 47, further comprisingdetermining which of the channels has the lowest power level on theoptical fiber.
 50. The method of claim 49, wherein a beam of lightformed from the lowest power level channel is coupled into another portwith about minimum attenuation.
 51. The method of claim 49, wherein apower of the predetermined fraction of the beam of light about equals apower of light from the lowest power level channel coupled into anotherport.
 52. The method of claim 47, further comprising multiplexing thechannels onto another optical fiber.
 53. The method of claim 47, whereinthe plurality of mirrors is a first plurality of mirrors, furthercomprising controlling an orientation of each of the first plurality ofmirrors to direct each of the beams of light against a separate one of asecond plurality of mirrors.
 54. An article of manufacture, comprising:a machine-accessible medium having associated data, wherein the data,when accessed, results in a machine performing operations comprising:demultiplexing the channels from the optical fiber to form a pluralityof beams of light, each beam of light formed from a separate channel;measuring the power level of each channel; directing each of the beamsof light against a separate one of a plurality of a mirrors; andcontrolling an orientation of one of the mirrors such that apredetermined fraction of the beam of light directed against the one ofthe mirrors is coupled into a port, the predetermined fraction less thana maximum fraction corresponding to optimal coupling into the port.